It has been known in the past to provide an exhaust purification catalyst such as an NOX storage reduction catalyst or oxidation catalyst in an exhaust passage of an internal combustion engine and to supply fuel as a reducing agent to the exhaust purification catalyst. For example, fuel is supplied by a fuel addition valve arranged an the exhaust passage at an upstream side from the exhaust purification catalyst.
To promote an oxidation reduction reaction in an exhaust purification catalyst, desirably the fuel injected from the fuel addition valve is made to vaporize or atomize inside the exhaust passage to raise the diffusibility of the fuel. To make fuel vaporize or atomize even if the temperature inside the exhaust passage is low, it is necessary to make the fuel injected from the fuel addition valve to the exhaust passage a high temperature. Further, to promote an oxidation reduction reaction at the exhaust purification catalyst, it is necessary to maintain the temperature of the exhaust purification catalyst at the activation temperature or more. However, if low temperature fuel is injected from the fuel addition valve into the exhaust passage, the latent heat of vaporization of the fuel causes the exhaust gas to fall in temperature and the exhaust purification catalyst to be cooled.
Therefore, to promote the oxidation reduction reaction at an exhaust purification catalyst, it is necessary to make the fuel injected from the fuel addition valve to the exhaust passage a high temperature. Regarding this, in the fuel addition system according to PLT 1, it is described to return the fuel heated by the heat of the internal combustion engine to the inside of a holding container inside the fuel tank and to supply the fuel inside the holding container to the fuel addition valve.